Global climate change is becoming a big issue. In the coming decades, it’s likely to be the defining global issue of our time – as big as the Cold War and the threat of nuclear war, and probably bigger. If the changes currently foreseen by science do happen, it will affect all of us, perhaps in wrenching ways.
W. R. (Bill) Prescott, guest editor for this issue, brought the magnitude of these changes home to us during a telephone call when he said, "There’s no way you can get to a humane sustainable culture if the current projections for climate change come true." We gulped, thought about it a minute, and asked Bill if he would be willing to work with us on this issue. He agreed, and we’ve enjoyed the partnership.
The premise of this issue is a scientific prediction: humans have changed (and are continuing to change) the chemistry of the atmosphere in ways that will eventually cause major changes in the Earth’s climate, including a general warming, the melting back of the polar ice caps, dramatic shifts in weather patterns, and perhaps much more. Never before has a scientific prediction been so critical to the world’s future.
Those of us who care about the world’s future are, sooner or later, going to have to learn a lot more about climate. The following article by IN CONTEXT editor (and former astrophysicist) Robert Gilman will introduce you to the complex – but fascinating – world of climatology. The articles that follow explore the new ways of thinking, living, and governing ourselves that global climate change calls us to discover.
It’s not yet clear if the Earth is getting hotter, but there’s certainly no question that the debate about climate change is heating up. Greenhouse critics and proponents are getting louder and louder. "Is it real?" "Will the Earth get warmer, or are we headed into another ice age?" "How fast will the changes come, and how serious will they be?" "Can climate change be stopped or will we just have to adapt?"
As we researched this issue and attempted to make sense of the questions it raises, I kept finding that the simple accounts of "global warming" in the popular press didn’t give me enough understanding to make intelligent judgments. So I went back to the scientific literature and the critics’ originals. In this article I’d like to share with you the essence of what I found.
We need to begin where the popular press usually ends, with a simple explanation of the "greenhouse effect." Essentially everyone agrees that:
1) The surface of the Earth is naturally warmer (by almost 60°F (32°C)) than it would be without an atmosphere because a few of the atmosphere’s gases – less than 1% by volume – are both transparent to visible light (which accounts for most of the incoming solar energy) and opaque to infrared heat radiation (the outgoing energy from the Earth into space). Thus the incoming energy has clear sailing right down to the surface, while the outgoing energy is partially blocked. This heat-trapping process is known as the "greenhouse effect" because glass has the same property.
2) The gases that cause this "heat trap" include water vapor, carbon dioxide (CO2), methane, chlorofluorocarbons (CFCs), and nitrous oxides.
3) Human activities – like the burning of fossil fuels and deforestation – have already increased the amount of these "greenhouse gases" (CO2 is up by about 25% from its pre-industrial level) and the rate of increase is growing each year.
THERE IS MORE TO CLIMATE CHANGE THAN THE GREENHOUSE EFFECT
Given only this description, it would be reasonable to assume that the climate should be gradually warming at a pace similar to the increase in greenhouse gases. Figure 1 (below) shows that it’s not that simple. We’ve plotted the observed atmospheric concentration of CO2 and average global surface temperature for the past 100 years. The CO2 curve rises smoothly, growing gradually steeper. The temperature curve also rises, but its shape does not match the CO2 curve at all. It rises most before 1940, actually declines from there to about 1965, and then heads up again.
Figure 1: Average global temperature (five year running mean)
and atmospheric CO2 (annual mean) since 1880.
What’s going on here? Does this lack of fit contradict the warming theory?
To understand climate change in the real world, we’ve got to get more sophisticated. Climate scientists have long been aware that there are many factors that can contribute to variations in climate. They include:
1) Variations in the energy output of the sun.
2) Variations in the Earth’s orbit.
3) Variations in the transparency of the atmosphere to incoming or outgoing radiation caused, for example, by volcanic dust, greenhouse gases, or variations in cloudiness.
4) Changes in the heat distribution and circulation of the oceans.
5) Changes in the reflectivity/absorptivity of the Earth’s surface due to changes in the distribution of vegetation, ice, etc.
These are usefully divided into forcing factors and feedback mechanisms. Forcing factors come from outside the climate system, like variations in the sun. Feedback mechanisms operate within the climate system and can either amplify or reduce the impact of some forcing factor. For example, there are large amounts of CO2 dissolved in the oceans (almost 50 times more than in the atmosphere). The amount of CO2 seawater can hold depends on its temperature: it holds more when cold, less when warm. So as human-produced CO2 (a forcing factor) warms the atmosphere, the oceans will also gradually warm, releasing still more CO2 (amplifying feedback).
There are two other feedback systems that are important for understanding climate change and the current debate around it. The first is the snow-and-ice/albedo/temperature feedback mechanism, another amplifying effect. Snow and ice are highly reflective (high "albedo"). They do not absorb as much solar energy as would the dark land or water under them, so they help to keep the air over them cool. Thus a little cooling, if it extends the range of snow or ice cover, can lead to a lot more cooling, and vice versa for a little warming. This process operates every fall and spring, sharpening the change of seasons. On a longer time-scale it is central to the growth and retreat of ice ages.
The second feedback effect involves clouds and is not as clear cut as the snow/ice effect. The uncertainty comes from the fact that clouds act both to cool and warm the earth. Like snow and ice, they are more reflective than the land or water below them, so in strong sunlight they act as a cooling agent. On the other hand, being made of water, they are opaque to infrared radiation, so they trap the heat radiating up from the Earth, like a blanket. The relative importance of these two opposing effects depends on subtle, hard-to-predict factors like the altitude, type, and height of the clouds. Clearly, clouds over the poles in winter keep things warmer since there is very little incoming solar radiation. Conversely, tropical clouds mostly cool, but the net effect of changes in cloud cover over the mid-latitudes is hard to predict.
CLIMATOLOGY IS AN INEXACT SCIENCE
Disentangling these various effects isn’t easy. In attempting to do so, climate scientists have used a two-pronged approach, one experimental and the other theoretical. On the experimental side, they are now making as many direct measurements of important quantities (such as actual solar energy output or the temperature distribution in the oceans) as possible. But in many cases these direct measurements extend back for only a decade or two, which is a very short time in terms of climate. To get useful data on the past, climatologists have had to be more indirect and ingenious. For example, they have used the gas bubbles trapped at different levels in the polar ice as a sample of past atmospheric composition. They have also compared the amount of warm-loving and cold-loving plankton remains at various depths of ocean sediment, and used this as a way of estimating past ocean temperatures. These and other "proxy" measurements are hardly perfect, but they do yield a surprisingly detailed picture of many climate variables going back often hundreds of thousands (and sometimes even billions) of years.
The main theoretical approach has been the creation of computer-based mathematical models for climate. Most important here are the general circulation models that attempt a reasonably accurate portrayal of climate in both time and space. These models can be tested against observed weather, and by now they are quite good at reproducing observed seasonal and geographic variations in climate. These models are also used to estimate how the climate would change if some forcing factor (for example the amount of CO2 in the atmosphere) were changed.
Computer models are currently our best tool for attempting to disentangle the relative importance of the competing factors that influence climate. However, it is important to understand the limitations of these models. First, their good fit to current climate has been obtained by tinkering with various equations within the models to make them fit. While modelers attempt to do this in as scientifically plausible a way as possible, there is still a certain amount of blind fitting. No one knows if this fitting would still apply with significantly different forcing factors.
Second, these models do not take all physical processes into account – and often only approximate those that are included. This is partly a conscious choice to reduce the complexity of the models (which still take hours of supercomputer time to run), but it also reflects the fact that we simply don’t know all the factors that could contribute to climate. Especially difficult are some of the more subtle feedback effects. For example, early climate models with fairly crude handling of cloud cover predicted an eventual global warming of 1.5° to 2.0°C, while more recent models with more sophisticated cloud handling predict a 4.0° to 5.2°C warming – more than twice as much.
PUTTING THE PIECES TOGETHER
Equipped with these various concepts and tools, we can now return for a more sophisticated look at Figure 1. We now know that it is naive to expect a good fit between CO2 and global temperature – there are just too many other factors involved. The variations in the curve could be due to 1) errors in measurement; 2) internal variations within the climate system; 3) variations in forcing factors. Greenhouse gases are only one effect of many. We need to ask, how important will the other effects be, and how much should CO2 be contributing if the warming theory is correct?
Errors in measurement are a potentially serious issue. This is not to say that climate scientists can’t read their thermometers. The problems are more systematic. For example, during most of the past 100 years there have only been a few weather stations in the southern hemisphere. How can we be sure that these provided an adequate sampling of that hemisphere’s climate? Another problem comes from weather stations in urban areas. Urban areas, being both heat producers and better heat absorbers, tend to be hotter than undisturbed countryside, with the size of the "urban heat island" effect growing for larger cities. Could this effect be responsible for the gradual upward drift of temperature? Hansen and Lebedeff, whose research is reflected in this chart, carefully considered these and other sources of error and found them all to be less than 0.1°C. The net result is that, while we shouldn’t take every wiggle of the chart too seriously, the overall pattern – a general rise until about 1940, a slight decline to about 1965, and a rise since then – is real.
Could these ups and downs just be random fluctuations or perhaps irregular oscillations within the climate system, not needing any "external" explanation? Nobody knows for sure. One clue comes from the computer models, some of which do show temperature variations (internal oscillations) of about +/-0.1°C that can last for decades. That’s not enough to account for most of the observed ups and downs, but enough to be a contributing factor.
Finally, what about the role of various external forcing factors other than CO2? The two most important candidates are dust and changes in the sun (variations in the Earth’s orbit occur too slowly to be significant over this short time span). Of these, atmospheric dust (mostly from volcanoes but increasingly from human activity as well) is the easiest to test and is estimated to account for about half of the temperature variation of the past few hundred years. More recently, the observed veiling by dust declined from 1880 to 1940 and then rose again at least through 1980, in close correlation with the rise and fall of temperature on either side of 1940.
Figure 2: Likely forcing factors for global temperature change. Upper curve — smoothed values for the optical depth of aerosol (mostly dust) in the Northern Hemisphere. Values decrease upward to illustrate the likely impact on global temperature. Lower curve — solar activity estimated from the smoothed envelope of sunspot numbers.
But dust isn’t the only source of variation. Solar variability has long been suggested as a likely candidate, but it is only recently that the full complexity of the sun-Earth interaction has begun to get disentangled. The connection is often subtle. For example, the sun exerts varying tidal forces that affect currents in both the oceans and the atmosphere, and the solar wind affects the Earth’s magnetic field and such things as ozone in the upper atmosphere. In addition, the energy output from the sun varies, at least with the solar sunspot cycle, and perhaps on longer time scales as well.
The importance of these effects is still controversial, but I’m convinced they need to be included. For multi-decade-long climate trends, the best correlation seems to be with the overall level of solar activity (flares, sunspots, etc.): high solar activity correlates with high global temperatures. How does this work during the past 100 years? From a low in the 1880s, solar activity has climbed to a peak in 1960, declined to another low in 1970 and has climbed since.
Where does this leave us? Taken together, dust and solar variations give a remarkably good fit to the general timing of ups and downs in global temperature from 1880 to 1980. But the overall upward drift during the past 100 years is not so easily explained. For this, we’ll have to look more closely at CO2‘s expected contribution.
Figure 3: Global temperature variations as observed (from Figure 1),
and as computed by combining the curves for dust, solar activity
(both from Figure 2), and CO2 (from Figure 1).
As mentioned earlier, the general circulation computer models predict that a doubling of CO2 leads to a temperature increase of from 1.5° to 5.2°C, with the higher range coming from more recent and supposedly more accurate models. Current temperature increases should be smaller for two reasons. First, today’s CO2 is only about 20% above the 1880 level. (Including the effect of other greenhouse gases like methane raises these to "equivalent CO2" levels of about 32%.) Second, even though the increase in CO2 in the models is instantaneous, the temperature rise doesn’t happen immediately. It takes years to warm the oceans, melt back ice caps, and do the other things that are involved in various amplifying feedback effects. In effect, today’s warming should correspond to the CO2 level of about a decade ago. Putting this all together suggests that the greenhouse gases should be making the global temperature in 1980 warmer than it was in 1880 by somewhere in the range of 0.3° to 1.0°C. The data indicate about a 0.7°C rise.
WE CAN’T WAIT FOR MORE DATA
Does this prove that a greenhouse-driven warming is now underway? The honest, scientifically conservative answer has to be no, just as a temporary cool spell brought on by extra dust or solar changes wouldn’t disprove the warming theory. There are just too many inadequately understood processes going on to definitively prove much of anything about climate.
But if we stop here we miss the point. If the warming is underway, it is not just a matter of academic interest. We don’t have the luxury to wait until more data comes in before we act. The more relevant question is, therefore, does the greenhouse effect provide the most plausible explanation of the observed upward temperature drift? Decide for yourself, but for me the answer is clearly yes.
If this rise is due to greenhouse warming, the implications are great indeed, because it is just the early warning sign of much more to come. You’ve no doubt read about rising sea levels and shifting climate zones. These are important and would cause major environmental and economic dislocation, but they may turn out to be only the most conservative side of what’s in store.
In the immediate future the more pressing concern is not the average weather but its variability. The greenhouse gases have pushed the climate system (including the oceans) out of equilibrium, and it will take a while for things to readjust. In the process all aspects of the system, like ocean currents, may change erratically. It is not clear yet what connection there is between, for example, the increased strength of El Niño/La Niña events in the Pacific Ocean and the rise in CO2, but certainly the 1980s have been characterized by increasingly erratic weather. The environmental and economic stress from this variability will hit us much sooner than the effects of gradual warming.
In the longer term, the big unknown is the strength of various amplifying feedback effects and the possibility of some instability in these. For example, current estimates are that the major human activities of burning fossil fuels and cutting down forests are putting about 7 billion metric tons (BMT) of CO2 into the air each year, but only 3 BMT is showing up as a net increase in atmospheric CO2. Where is the rest of it going? Apparently into the ocean. Natural processes – respiration by plants and animals, photosynthesis, and diffusion at the sea’s surface – cycle about 200 BMT in and out of the air each year (about 30 times the human rate!). The land processes are closely balanced, but the ocean is currently absorbing 104 BMT each year while only releasing 100 BMT. How long will the ocean keep absorbing more than half our waste CO2? No one knows, but the ocean process is temperature dependent – warm water releases more CO2 and absorbs less, so if the oceans start seriously warming we could have a very rapid rise in CO2.
Even worse, if some of the natural biological processes that now absorb vast quantities of CO2 were to be disrupted – massive forest death from the stress of climate change for example, or the die-off of the ocean’s phytoplankton – the increase in CO2 could jump by many times its current rate.
I don’t want to be alarmist, and I’m certainly not saying that these things will happen, but they are well within the range of possibility. In any case, unless we change course soon, we will be causing massive changes in the climate at a pace much faster than most living systems can adapt. Within the next century, and perhaps even the next few decades, we could face much more than just the "inconvenience" of losing a little coastline. We could face an environmental catastrophe leading to the extinction of most species alive today, and the decimation of our own.
by Robert Gilman
Perhaps the best known alternative to the usual global warming prediction is the ice age theory put forward by John Hamaker in the late 1970s. I briefly described this theory in IN CONTEXT Issue #9 (Spring 1985) and have been following the controversy around it since. Given the serious implications of major climate change, it makes sense to look at this theory again.
In its totality Hamaker’s theory is quite comprehensive, encompassing the interaction between climate, soils, vegetation and volcanic activity on a geologic time scale. Central to the theory, however, are the ideas that 1) progressively depleted soils lead to less trapping of CO2 by vegetation and 2) increased CO2 leads to expanded snow and ice cover at the poles. This theory is intended to provide an entirely natural explanation for the onset of ice ages; present human activity is just seen as accelerating the process.
There are many points where this theory can be (and has been) disputed, but for our present purposes the key controversy lies around the question, does increased CO2 lead to more polar snow and ice cover, or less? Hamaker’s reasoning goes (in part) like this: The heating from the greenhouse effect should be strongest where the sun is strongest, namely in the tropics, and weakest at the other extreme, namely the winter pole. This should increase the difference in temperature, driving a stronger circulation of air between the tropics and the winter pole. At the same time, the extra warmth in the tropics will evaporate more moisture from the oceans. The net result would be more moisture carried to the winter pole where it will form clouds and precipitate out as snow. The increased snow cover will in turn keep the pole cooler all year, leading to still greater snowfall. Before long, the tropical greenhouse has led to a runaway polar ice cap and a new ice age.
All the computer models show just the opposite effect. The poles, both winter and summer, show the largest increases in temperature, and the ice caps shrink. The basic physics behind this is straightforward. The only thing that keeps the winter pole warm at all is its greenhouse blanket. Without the atmosphere, the sun-heated tropics would be only a few tens of degrees cooler, but the winter pole would be hundreds of degrees colder. Without the greenhouse effect, the warmth coming to the winter pole via air and ocean currents from other places would immediately radiate away into space. Thus, increases in the greenhouse effect have a larger effect on temperature at the winter pole than in the tropics; that is, they lead to a more uniform global temperature.
Hamaker’s response is that the models aren’t realistic; in particular that they don’t adequately handle the role of clouds. However, as the cloud handling in the models has grown more sophisticated and better able to reproduce existing climate conditions, the predicted heating has increased, not decreased. Nevertheless, there is still enough uncertainty surrounding the models that they can not be considered definitive proof against Hamaker’s theory.
The other way to test the theory is against the observations. Since Hamaker first proposed his theory much has been learned about both recent and longterm climate history. The deep Antarctic ice cores give a record for both CO2 and temperature going back 160,000 years, long enough to include all of the most recent ice age, the warm interglacial period before it, and into the previous ice age. The overall result is that CO2 and temperature generally move together, in agreement with the standard computer models and not with Hamaker’s theory.
Climate history based on Vostok ice core. CO2 is directly measured, the observed temperature is based on deuterium concentrations in the ice, and the computed temperature combines the effects of CO2 and changes in the Earth’s orbit (see text). From Lorius et al.
The ice core record shows a small rise in CO2 (less than 4%) just as the last ice age began. Ephron, one of Hamaker’s supporters, has suggested that this supports the theory, but it is hard to see how such a small increase (comparable to the normal annual variation in CO2) could by itself set off an ice age when even higher levels of CO2 earlier in that interglacial period are associated with the warmest climate – and throughout the 160,000 year record, increases in CO2 correlate very well with rising temperatures. A more plausible explanation is provided by Lorius et al. who have shown that the temperature data from the Antarctic ice cores can be duplicated with remarkable accuracy by a model that includes the effect of both CO2 and changes in the Earth’s orbit. From this perspective, the bump in CO2 at the end of the last interglacial slowed the decline in temperature rather than set it off.
Hamaker and his followers also cite the decline in global (and particularly northern hemisphere) temperatures from 1940 to the 1970s as evidence for a "cold pole" greenhouse effect, but they have no consistent explanation for the increase in temperature before 1940 or in the past two decades. A good theory doesn’t have to pick and choose the observations it will explain. As I’ve described in the accompanying article, the whole 100-year pattern can be plausibly explained by atmospheric dust, solar variability and greenhouse warming.
These are just two examples; I could go on, but in all the instances I’ve been able to check, I’ve found the same story. The observations which they claim support the theory always have a more plausible alternative explanation consistent with the standard idea that CO2 leads to warming.
Having said all this, I also want to say that I respect the scope and ingenuity of Hamaker’s theory, and I am well aware that much of the material that now makes this theory look so doubtful was not available when Hamaker first proposed it. As often happens with interesting theories that turn out to be wrong, it has stimulated its adherents to ask valuable questions and to develop useful perspectives: time is short, the coming changes could be massive, and one good way to rapidly increase the CO2-capturing power of vegetation is to improve the health of the soil. Indeed the emotional heart of Hamaker’s theory seems to be his program for rebuilding the soil by adding minerals to it in the form of rock dust. It is a good idea, deserving of much more attention than it has gotten so far.
Theories are often like a scaffolding – they allow you to build something that you could not have built without them, but the time comes when the new building must stand on its own. My hope is that Hamaker and his followers will now let the urgency of soil remineralization stand on it own. It is as important and as urgent in a warming world as in a cooling one.
Ephron, Larry, The End; The Imminent Ice Age & How We Can Stop It, (Berkeley: Celestial Arts, 1988).
Hamaker, John D. and Weaver, D.A., The Survival of Civilization, (Burlingame, CA: Hamaker-Weaver, 1982).
Hansen, James, and Lebedeff, Sergej, "Global Trends of Measured Surface Air Temperature," Journal of Geophysical Research, 1987, Vol. 92, No. D11, p. 13345.
Houghton, R.A., and Woodwell, G.M., "Global Climatic Change," Scientific American, April 1989, p. 36.
Lamb, H.H., Climate, History, and the Modern World, (London: Methuen & Co, 1982).
Lorius, C. et al., "Antarctic Ice Core: CO2 and Climatic Change Over the Last Climate Cycle," Eos, June 28, 1988, p. 680.
Rampino, M.R. et al., eds., Climate: History, Periodicity, and Predictability, (New York: Van Nostrand Reinhold, 1987).
Schneider, Stephen H., and Londer, Randi, The CoEvolution of Climate and Life, (San Francisco: Sierra Club, 1984).
Tucker, Peter, ed., Solaris Bulletin, February/March 1989 (Solaris Research, Fack, 170 11 Drottningholm, Sweden).
Wilson, C.A., and Mitchell, J.F.B., "A Doubled CO2 Climate Sensitivity Experiment With a Global Climate Model Including a Simple Ocean," Journal of Geophysical Research, 1987, Vol. 92, No. D11, p. 13315.